La vuelta a España / The tour of Spain

Yesterday finished "La vuelta a España" finished in the centre of Madrid, more specifically in Cibeles Square close to the city hall. Finally the Italian rider Fabio Aru won the general classification and could wear the red jersey on the top of the podium, flanked by the Spanish rider Joaquim Rodriguez as second-placed (or in second place) and the Polish Rafal Majka as third-placed.
The This past Saturday three friends and I were cycling to Navacerrada mountain pass to watch the professional cyclists when they go through the mountain banner(?). There were a lot of supporters near the mountain banner(?) so we went a bit further, where the road begins its descent. There we could see and encourage the cyclists better than in at (or on) the crowded top of the mountain pass. It lasted only a few seconds but it worth the effort to climb the mountain with our bikes. First passed the breakway group passed (or first to pass were the breakaway group) and minutes later the pack in which was the back the then leader of the race Tom Dumoulin was at the back.
At the end the Spaniard cyclist (or the Spanish cyclist) Ruben Plaza won the stage after a long solitary breakaway of 140 kilometers crossing alone the finish line alone in Cercedilla.

Very good Agustín! Mostly just a few corrections to the order of words. My corrections are in red and my suggestions to sound more natural are in blue/grey.

"the then leader of the race Tom Dumoulin" might sound a little bit strange, even though I think it's correct. Another way to say it would be "the leader of the race at the time, Tom Dumoulin, was at the back".